The present invention pertains to the method of production of ethylene copolymers suitable for manufacturing films and pipes using supported catalysts containing chromic acid esters.
Basic types of catalysts based on chromates provide mostly polymers with low melt flow rates and broad molecular weight distribution. These catalyst systems for ethylene polymerization were disclosed in U.S. Pat. Nos. 3,324,101 and 3,324,095. According to the quoted patents, the supported catalyst system is prepared as follows: a silyl chromate compound is anchored to the surface of a support, typically activated silica (being activated at a temperature between 100 and 900xc2x0 C. for 2 to 6 hours) in an aliphatic hydrocarbon, then an organoaluminium compound is added at an Al/Cr ratio between 0.1:1 and 15:1. According to the examples presented in the above patents the catalyst produces, at an ethylene pressure of 1.33 atm at a temperature between 60 and 90xc2x0 C. and at a hydrogen pressure of 1.3 atm, a polymer with melt flow rate in a range of 0.04-1.57 dg/min (at 21.6 N load) and exhibits a very broad molecular weight distribution reflected in MFR216/MFR21.6 between 100 and 170. If a 1-alkene comonomer is used, a copolymer is prepared, exhibitingxe2x80x94depending on polymerization conditionsxe2x80x94density in a range of 0.938-0.956 g/cm3. The polymer thus prepared is probably designed to be used for manufacturing films or pipes employed in distribution systems for hot and cold water and pressurized natural gas.
In EP 0 454 434 Patent a procedure for preparing a new silyl chromate component was disclosed which is used for preparation of a catalyst system employed for polymerization of 1-alkenes. The procedure of the catalyst preparation, including the polymerization condition, is identical to the above-quoted U.S. Pat. Nos. 3,324,101 and 3,324,095. The catalyst produces (at 95 C and in the presence of hydrogen) a polymer with melt flow ratio in the range of 1-6 dg/min. and exhibits a medium-to-very wide molecular weight distribution.
In CZ 285 446 Patent a procedure for preparing polymers with a high melt flow rate and a narrow molecular weight distribution was disclosed using a catalyst system containing a silyl chromate component. A principle of the procedure is a modified method of preparation of the catalyst in which the type and concentration of hydroxyl groups on the silica surface provide formation of active centers with linear structure. This type of active center prefers transfer reactions on account of copolymerization, if 1-alkene monomers are polymerized.
Catalysts based on chromates produce polymers with a high content of internal Cxe2x95x90C double bonds (vinylidene, vinylene) and low content of so called xe2x80x9ctie moleculesxe2x80x9dxe2x80x94branch polymer chains of high molecular weightxe2x80x94providing good long-term mechanical properties. Taking account of these shortcomings, the polymers are unable to meet increasing demands on polymer quality; therefore the polymers for more demanding applications are prepared using new types of catalyst systems or novel procedures for polymer preparation. These systems or polymerization procedures can control more efficiently the structure of the polymer matrix. The catalyst systems belonging to this group should ideally be chromium oxide systems; the basic version was disclosed in U.S. Pat. No. 2,825,721. A novel procedure of the polymer preparation is a process with a cascade of two reactors where, e.g., a slurry polymerization in the first reactor is followed by a gas-phase polymerization in the second reactor.
According to the present invention, a procedure of preparation of ethylene copolymer suitable for film or pipe applications is provided, using a substantially innovated catalyst system containing chromic acid esters. Utilization of this catalyst system allows preparation of copolymers with a high content of high molecular weight chains, with an optimum degree of branching providing an efficient interlining of crystalline and amorphous phases of polymer. According to the present invention, we have found a novel efficient procedure of preparation of a suitable type of active catalyst able to prepare the type of polymer required. The principle of preparing a copolymer suitable for manufacturing pipes or films consists in modification of conditions of the active center formation selected so that after interaction of:
a) silyl chromate of general formula I being suspended prior to contact with a support in an aliphatic hydrocarbon in the temperature range of 0-80xc2x0 C. and bubbled by an ultra-pure nitrogen 
wherein R1 is an aliphatic, cyclic or aromatic hydrocarbon group containing from 1 to 20 carbon atoms,
b) porous supporting material with surface acidic hydroxyl groups, their concentration and character being modified prior to the interaction by thermal treatment,
an intermediate was formed in a high yield of a structure A 
as a result of hydrolysis of two ester groups in silyl chromate exclusively by hydroxyl groups of the support and the intermediate A after reduction by
c) alkylating agent, an organoaluminium compound of general formula II
R2aR3bAl(OR4)3xe2x88x92(a+b),xe2x80x83xe2x80x83(II),
combined with an organomagnesium compound of general formula III
xe2x80x83R2R3Mg,xe2x80x83xe2x80x83(III),
wherein R2, R3, and R4 are aliphatic, cyclic or aromatic hydrocarbon groups containing from 1 to 20 carbon atoms, said hydrocarbon groups being the same or different, a and b are integers from 0 to 3,
provided an active center with an enhanced ability to enchain a 1-alkene comonomer, particularly in the high molecular weight fraction of polymer, as compared to the existing catalysts,
while ratios between the components in the catalyst vary in the following ranges: content of silylchromate between 0.05 and 1.0 wt. % of the support, the ratio between total amount of organometal (ORG) to silyl chromate, expressed as ORG/Cr varies between 0.5 and 20 and the ratio between organoaluminium and organomagnesium compounds expressed as Al:Mg varies from 10:0.1 to 0.1:10.
In a preferable procedure for preparing the catalyst system according to the present invention, the support is solid porous material, such as silica.
In a preferable procedure for preparing the catalyst system according to the present invention, the silica activation process is conducted so that apart from the isolated hydroxyl groups the support also contains vicinal ones, the latter OH groups making possible formation of the species depicted in Structure A. This structure after alkylation is a source of polymerization active centers.
In a preferable procedure for preparing the catalyst system according to the present invention, it is highly desirable to avoid the presence of water, particularly during the interaction of chromate with hydroxyl groups of silica.
In a preferable procedure for preparing the catalyst system according to the present invention, the silyl chromate compound is alkylated by ROxAlR3xe2x88x92x first and then by R2Mg compound or simultaneously by their mixture.
In a preferable procedure for preparing the catalyst system according to the present invention, the organoaluminium compound is Et2AlOEt and the organomagnesium compound is butyloctylmagnesium.
The advantages of preparation of the catalyst system according to the present invention follow from the fact that the selected procedure of the catalyst system preparation provides active centers in a high yield, the centers utilizing a comonomer preferentially to short-chain branching, particularly in the higher-molecular-weight polymer fraction. This, in turn, decreases occurrence of transfer reactions with a comonomer resulting in a lower content of unsaturated Cxe2x95x90C double bonds and thus, in a lower appearance of low molecular weight highly branched polymer chains. These changes are beneficial in limiting undesirable reactions during polymer processing. The catalyst containing organomagnesium compound exhibits higher activity and higher stability during the polymerization process.
The advantages of the preparation of ethylene copolymers according to the present invention in comparison with the standard procedure are illustrated in the diagrams (FIGS. 1 and 2) where the copolymer structure is presented as fractions with a certain molecular weight and certain number of branches. The abbreviation xe2x80x9clog Mxe2x80x9d means the decadic logarithm of molecular weight, while xe2x80x9cCH3/1000Cxe2x80x9d means the number of branches per 1000 carbon atoms in the polymer backbone.
Further advantages of the catalyst system according to the present invention are seen in the simple and fast procedure of the catalyst system preparation both in the laboratory and in a commercial plant. Also, standard methods and facilities can be used. The polymerization activity of the present catalyst system is very high. The catalysts react sensitively to the comonomer employed and, hence, polymers with widely varying density can be prepared.